An interesting idea to be sure.  I always hoped that the next Ranma 1/2
fighting game (if they ever make another one) would be available here
for Playstation/Saturn/N64/3DO/etc. (whatever system you prefer) but
thanks to DTMC's bad voiceovers of the SNES American Release, it may
never see the light of day here.

The three fighter teams with a combo system like Street Fighter Alpha 2
would be cool.  Herb, Toma and Suffron could be the bosses with an
original final boss at the end.

The regular fighters would use the moves they learned from the
manga/anime. Ranma/Happosai/Cologne would have to be cut down a bit in
power to have an equal balance for all fighters.

Likewise, a reason for the fighters to fight....

Ranma/Ranma-Chan: Promised yet another cure for his/her curse. 
Ryouga: Cure his sense of direction so he can finally find Akari.
Mousse: One word: SHAMPOO!!!
Shampoo: One word: RANMA!!!
Gosunkugri: Two words: AKANE TENDOU!!!
Akane: Hmm...learning to cook has been done....what do you suggest?
Genma-Panda: To be free of his evil master at last.
Happosai: To force Ranma-Chan to wear his bra at last.
Kunou: The death of the evil sorceror Saotome.
Kodachi: Normally Ranma, but now that she's aware he doesn't love her...
Ukyou: Too many want Ranma....How about vengence on Genma?
Cologne: Force Ranma to marry Shampoo.
Hinako: Prove herself to Soun Tendo.
Sasuke: Who knows?!?
Soun: (See Genma)

(Soun may be an ordinary fighter but could serve as the middleman.)

(etc, etc, whoever else you want to fit in....)

In addition a secret character, simular to the dream match fighter from
Fatal Fury Special, would be good.  My first choice would be Lum from
Urusei Yatsura.

Name: LUM

Story: The princess of Oniboshi and husband of resident lech Ataru
Moroboshi, she is mislead by Nabiki into thinking her darling is lurking
somewhere in the tournament and seeks to find him by any means
nessecary.

SUPERHUMAN POWERS: Lum possesses the ability to bodily generate 50,000
volts of electricity in a varity of forms as well as the ability to
float and attack from the air.    

WEAPONS: LIGHTNING FROM HER BODY AND FINGERS

SPECIAL ATTACKS: 

"LIGHTNING BALLS" (LUM FORMS A SMALL GREEN BALL OF ELECTRICITY IN HER
HAND AND THROW THE ENERGY BALL AT THE OPPONENT.) 

"MAXIMUM VENDICTIVENESS" (LUM CHARGES HERSELF UP FOR A MOMENT AND THEN
UNLEASHES A STREAM OF ELECTRICITY AT THE FLOOR WHICH RUSHES ACROSS THE
GROUND TOWARDS THE OPPONENT, SHOCKING HIM/HER IF IT HITS.) (MUST BE DONE
WHILE FLOATING)

"DEVINE RETRIBUTION" (WHILE LUM IS FLOATING SHE RAISES HER HANDS OVER
AND BEHIND HER HEAD AND SHOOTS THREE QUICK BLASTS OF ELECTRICITY AT THE
OPPONENT.)

"AIR SIDE KICK" (WHILE LUM IS FLOATING SHE DIVES DOWN AT THE OPPONENT
WITH HER LEG WHILE BODILY EMITTING ELECTRICITY INTO HER BODY.)

"FLOAT" (ACTS JUST LIKE SINDEL'S FLOAT MOVE FROM MORTAL KOMBAT 3 ONLY
WITH MORE FREEDOM OF MOVEMENT.)

LOCATION: ? (A landmark from UY perhaps?)

MUSIC: "Rock the Planet"?     

BEGINNING STANCE: LUM'S UFO APPEARS IN THE SKY FOR A MOMENT AND A BOLT
OF LIGHTING STRIKES THE GROUND REVEILING LUM ASSUMING A BEGINNING STANCE
SIMULAR TO RAYDEN'S FROM "MORTAL KOMBAT" THE ELECTRICITY PULSATING ALL
OVER HER BODY.    

VICTORY POSE: Dunno, but Lum must say something that ends with "Da
Cha!")

ULTIMATE FINISHING TECHINQUE: 

"ULTRA SUPER ELECTRIC ATTACK" (LUM'S BODY CRACKLES WITH ELECTRICITY
WHILE SHE MAKES A SERIES OF POWER UP MOVEMENTS WITH HER ARMS AND THEN
UNLEASHES A ENORMOUS AMOUNT OF ELECTRICITY AT THE OPPONENT FRYING
HIM/HER.) 

Anyone else got any ideas?